a fee cannot be a sole criterion for selection of the consultant - if you pay a relatively much lower fee, you will probably not get what you really need: there are a lot of 'consultants' that do (are able to do) only a small fraction of what is required and,
as a result, they can propose much lower fees than others.

MAKE SURE THAT QUOTATIONS  FROM ALL CONSULTANTS CONTAIN THE COMPLETE (ESSENTIAL) SCOPE OF WORKS, AND ONLY THEN COMPARE THE FEES - COMPARE APPLES WITH APPLES

  an engagement of a large consulting firm does not guarantee the satisfactory service, unless you know who will actually perform the job - if you pay less or have a smaller project, a chance is that you will end up with junior, inexperienced staff.

FIND OUT  WHO  WILL EXACTLY CARRY OUT THE JOB FOR YOU - IT DOES NOT REALLY MATTER, WHETHER YOUR CONSULTANT IS FROM A LARGE FIRM THAT HAS PERFORMED, AS A WHOLE, A NUMBER OF DIFFERENT PROJECTS - IT RATHER MATTERS WHAT HE OR SHE HAS DONE PERSONALLY - REMEMBER THAT NOT ALL EMPLOYEES OF
A SUPERMARKET ARE COMPETENT IN THEIR JOBS

  there is a number of other aspects that you need to take into consideration,
while choosing your acoustical and/or environmental consultant, viz.
(a)    formal qualifications;
(b)    status, registration and affiliation with professional institutions and associations;
(c)    wide-ranging (beside consulting, e.g. industrial, design, research) experience;
(d)    abilities to quickly recognize and effectively resolve technical problems;
(e)    quality of tools (measuring equipment, specialist software); and
(f)     professional indemnity (insurance); etc.

GATHER ALL RELEVANT INFORMATION ABOUT PERSONS THAT WILL BE POTENTIALLY
YOUR CONSULTANTS (IF SUCH INFORMATION IS NOT AVAILABLE, LOOK FOR OTHER CONSULTANTS THAT PROVIDE SUCH INFORMATION) AND ONLY THEN MAKE YOUR CHOICE.